New template function: wp_list_bookmarks()

Description

wp_list_bookmarks() will replace get_links_list() and get_links(). By default, it should create a list of all bookmarks organized by category. We want to avoid the mistake of hard coding h2s and other formatting, so there should be options for specifying any formatting that should come before and after each category heading. wp_list_bookmarks() should also allow creating a big list of uncategorized bookmarks as well as listing bookmarks in a specific category or set of categories. A stub for wp_list_bookmarks() exists in template-functions-bookmarks.php along with some implementation notes. wp_list_bookmarks() should use get_bookmarks() for querying the DB.